A marine vessel is impelled in the sea by a propeller which is attached to the engine from one end and to the other end of a shaft connected to a transmission. The extent of the shaft torque is substantially influential on the movement capacity of the marine vessel, since it accelerates the propeller. The output torque of the shaft is a function of engine power, speed and transmission reduction. In addition, shaft torque may not be at a satisfactory level for the thrust expected from the propeller. This is referred to as a “heavy” propeller in that the engine may not reach to the highest revolution power and figures, and for some, even some percentage (%) missing engine revolution may be acceptable, which means an increase in fuel costs and wear of the engine.
One of the solutions proposed for providing a better propeller thrust with the same engine power is to use a dual counter-rotating propeller structure, wherein the propellers are coaxially mounted on the shaft. The dual propeller structure is essentially based on the principle that the torque that is received from the shaft connected with the engine is transferred to a second shaft, with which the front propeller is in communication by means of a first power transfer means (for instance a gear wheel mechanism), and that the torque that is received from the shaft connected with the motor is transferred to a third shaft that is coaxial with the second shaft, the third shaft being in communication with the rear propeller by means of the second power transfer means. An exemplary arrangement for a dual propeller structure is disclosed in U.S. Pat. No. 6,821,169. Likewise, U.S. Pat. Nos. 6,702,631 and 6,478,641 disclose a dual propeller structure.
In the dual propeller structures mentioned in the above documents, the resultant thrust slightly increases as the torque that is necessary for rotating both propellers is provided by one power source, by the shaft which is in communication with the engine. This is because of the fact that two separate propeller devices entail two separate sources of energy loss. Thus, each propeller device has its own power transfer means, and they cause significant mechanical losses. Moreover, the rear propeller uses the water which has already been de-energized by the front propeller, causing a loss of efficiency.
On the other hand, U.S. Ser. No. 14/460,923 of the present inventor describes a turbo propeller system which is locked in one direction dependent on an engine shaft carrying the main propeller and rotates freely in the other direction. A turbo propeller is mounted with bearings that are locked in one direction and being free to rotate in the other direction. Thus, in the forward movement of the vessel, second propeller bearings are locked and water rotates the second propeller with a new, calculated pitch and provides extra torque gain to the main shaft. In rearward movement of the vessel, the turbo propeller rotates idly. The adaptation of the solution proposed in U.S. Ser. No. 14/460,923 to a conventional propeller that is already in use may involve relative challenges.
Another example of a coaxial propeller is propellers known as “Grim wheel” or “Vane wheel”. In this type of propellers, the rear propeller has, in general, a larger diameter than that of the front propeller and while the inner part of the rear propeller acts mainly as a turbine, and the outer part has a thrusting function. However, Grim or Vane wheel propellers do not provide any torque contribution to the shaft, and they just provide contribution to the thrust.